Telescopes and Lenses

24 02 2009

This is an exciting time in which to become an amateur astronomer. Never before have novice stargazers been presented with such a vast array of telescopes and accessories to pursue their hobby. Naturally, this brings the burden of choice. A bewildering variety of instruments make it difficult for the uninformed consumer to make the right decision. That’s what this entry is all about – to explain the options so you can choose the telescope that’s best for you.
Before examining the different types of telescopes available, it’s worthwhile illustrating some basic principles in order to understand how they work. The most important aspect of any telescope is its aperture, or the diameter of its main optical component, which can be a lens or a mirror – the fatter the tube, the larger the aperture. A scope’s aperture determines its light-gathering ability and its resolving power (the ability to see fine detail in an image).
What does this mean in real terms? With a 6-inch telescope, you can discern lunar craters that are as small as a mile across, which is half the size of those visible in a 3-inch scope under similar conditions. However, the same two instruments turned toward a faint galaxy on a moonless night would tell a different story. The 6-inch gathers four times the light of a 3-inch, not twice, so the galaxy would be four times brighter in the larger instrument. How is that? A 6-inch telescope has four times the light-collecting area of a 3-inch.

Power Isn’t Everything
It may surprise you, but a telescope’s aperture has no bearing on its magnification (or power). It’s the instrument’s focal length – the distance from the lens or mirror to a point at which it forms an image of a distant object – that governs magnifying power. When seeing an astronomical telescope for the first time, a novice will invariably ask: ‘How much does it magnify?’ The fact is that a telescope can provide an almost infinitely variable range of magnifications depending on the eyepiece used with it. Two main factors limit the power we can use on any given instrument: aperture and atmospheric conditions.

There’s a finite amount of detail in the image produced by a telescope’s main mirror or lens. We must find the optimum magnification to extract all of this detail without overly spreading out your target’s precious light, and making it too dim to see clearly. This is generally why low powers are employed to look at faint subjects like galaxies and nebulae. Just as enlarging a photograph too much will reveal the grain of the negative, so too will excess magnification make your target grainy.

How much power is too much? Fortunately, there’s a simple rule that we can apply to find top magnification: 50 times your telescope’s aperture in inches, or twice its aperture in millimetres. This means a high-quality 4-inch (100-mm) scope should not be pushed beyond about 200x. To put this in perspective, the smallest instrument that has good optics will show you Saturn’s rings or the principal cloud belts on Jupiter, since these can be seen at a magnification of 75x. On the other hand, if you see a 60-mm department-store scope labelled as capable of delivering 300x, you’ll know it’s advertising hype and will wisely move on to the next instrument!

Learning to See
Even with the best telescope, you’ll notice that you can discern finer lunar or planetary detail on some nights than on others. If you look closely in the eyepiece you’ll see that planets and stars appear to shimmer and waver on these less-than-perfect nights. The fault then lies not with the observer or the instrument but with the Earth’s atmosphere through which we look and local conditions, such as hot pavements. Astronomers refer to turbulent nights as having bad ‘seeing.’

Larger apertures are affected more than smaller ones by poor seeing, since they are capable in its absence of discerning finer details on the Moon and planets. This is why large telescopes – those in the 10-inch-plus category – are limited to 250 or 300x on all but the very steadiest nights. Not only that, any experienced observer will tell you that with constant practice you’ll see more detail in an image.

Is Bigger Always Better?
Why go for a telescope larger than 10-inch aperture if the sky conditions will limit you? Large apertures are most often chosen by observers who want to gather as much light as possible for viewing dim galaxies, nebulae and star clusters. These so-called ‘deep-sky’ objects are most often viewed at much lower powers than the Moon or planets, so seeing quality is less of an issue. In addition, larger aperture generally leads to shorter exposure times for those interested in astrophotography.

Not only might larger instruments be beyond your budget, but also there is the equally important question of portability. A large scope in amateur hands either requires housing in a permanent observatory, or the services of some willing observing buddies to move and assemble it for each observing session. Clearly, there’s a trade-off between convenience and performance, and everyone will have their own definition for what is portable. It’s so easy for the novice to succumb to the all too prevalent malady of ‘aperture fever’, where he or she is seized by a compulsion to buy the largest telescope they can afford. Any thoughts of portability are frequently discarded. The sad fact is that the leviathan is all too often consigned to the basement having been judged as being too unwieldy or heavy, and the novice astronomer goes off to pursue another hobby.

Scopes of Every Size and Shape
Having gained an appreciation of a few important optical principles governing telescope performance, now we can explore the different types of scopes available. You’ll be forgiven for thinking there’s a seemingly infinite number with a quick perusal of the advertisements in this publication. Yet for their varied shapes and sizes, all scopes can be divided into three distinct classes: refractors, reflectors and catadioptrics.

Refractors
A refractor is the instrument that conforms to the stereotypical image of a telescope – a long, gleaming tube mounted in a large dome with a lens at one end and a rather myopic astronomer in a white coat squinting into an eyepiece at the other. That’s not quite the picture, but captures essentially what they are like. Refractors have been with us in their various forms for 400 years and are often sought out by lunar and planetary observers who value their crisp, high-contrast views that can take high magnifications. In fact, when well made they can provide the finest images attainable for the aperture.

The other advantage of the refractor is that it’s generally more rugged than other types of scopes, its lenses less likely to come out of alignment. For this reason they are well suited to those who wish to have a ‘pick up and go’ instrument or have no desire to tinker with the optics.

So is a refractor the perfect scope? The downside is that these convenient features come at a price. A fine objective lens is a work of art that requires special glass and exquisite optical crafting to perform well. For this reason, refractors are the most expensive instruments of any given aperture. In their commonly encountered forms, the tube length can be unwieldy – sometimes ten to 15 times the size of the diameter of the lens – so a 4-inch scope could be 4 feet long. Add the fact that the eyepiece is at the lower end of the tube when looking at objects that are overhead, then some form of tall tripod is required in order for the observer to get underneath. This has to be solidly built to prevent vibration at high powers, so it may be heavy or unwieldy. For deep-sky observers there may not be the light grasp for viewing faint objects and the fields of view may be quite narrow. Modern design has led to shorter, faster, more manageable refractors, but at a correspondingly higher cost.

It’s All Done With Mirrors – Reflectors
The second type of telescope, the reflector, uses (as its name suggests) mirrors to gather and focus light from the object under scrutiny. In its most commonly encountered form – the Newtonian, which has been around for in excess of three centuries – there’s a specially curved concave (dish-shaped) primary mirror at the bottom end of the telescope. Near the top of the tube, a small inclined secondary mirror directs the light reflected from the primary to the side of the tube where it’s met by a conveniently placed eyepiece. If you want the largest aperture for your money, then the reflector is unquestionably the scope for you. When well made and maintained they can provide sharp, contrasty images of all manner of celestial objects at a small fraction of the cost of an equal aperture refractor.

The tube of a Newtonian is considerably more manageable, too. Their lengths are rarely eight times the diameter of the primary mirror and frequently less so. This means an 8-inch Newtonian can be housed in a tube only 4 feet long – easily fitting along the back seat of a small car for transportation to dark, rural skies. Combine this with the Newtonian’s generally low centre of gravity and you end up with an instrument on a compact, stable mounting, presenting the eyepiece at a convenient height for just about any sky orientation. For the best value scope of all, much consideration should be given to the Dobsonian form of the Newtonian. These extremely popular instruments are available in apertures from 4- to 30-inches plus and represent the ultimate in observer convenience for casual viewing, particularly if you’re not interested in taking photographs through it. (More on mounts later.)

Is there a downside? Like all reflectors (there are other types, but we’ll not consider them here because they tend to be specialist instruments not commonly encountered in amateur hands), Newtonians will require occasional maintenance. Unlike a refractor’s solidly mounted lens, a reflector’s mirrors can go out of alignment and hence will need periodic adjustment (collimation) to ensure peak optical performance, particularly if the telescope is moved frequently. This should not be overstressed, since the mirrors of the average Newtonian will not require tweaking for many months at a time. But for those not mechanically inclined, having to collimate a reflector even occasionally may be a frustrating or undesirable proposition. The open nature of the reflector’s tube means that dust and dirt is more likely to accumulate on the optical surfaces that will need periodic cleaning. Also, the aluminised surfaces of a reflector’s mirrors will need recoating every ten years or so, and more frequently in air-polluted urban areas or by the sea.

The Best of Both Worlds – Catadioptrics
Then there’s the third category of telescope, the catadioptric, or compound telescope as they are occasionally referred. These came about in the 1930s out of a desire to marry the best characteristics of refractors and reflectors. This is why they employ lenses and mirrors to form an image. The greatest appeal of these instruments is that in their commonly-encountered forms (the Schmidt-Cassegrain and Maksutov-Cassegrain) they are very compact – their tube lengths are two to three times the aperture of the scope due to the ‘optical folding’ of the light passing through them. The smaller tubes dictate more manageable (and consequently lighter) mounts and tripods. The practical upshot is that you can obtain a large aperture and a long-focus telescope that’s very transportable.

So is the Schmidt- or Maksutov-Cassegrain the ideal scope? As always, there are caveats. Like the Newtonian, the Schmidt-Cassegrain needs very occasional optical collimation that lessens its appeal to those disinclined to tinker. Their fields of view can be quite narrow, too. In terms of cost, aperture for aperture, the catadioptric lies midway between the reflector and the refractor. Like a Newtonian, the popular forms of this compound telescope have secondary mirrors that lie in the light-path of the instrument that slightly degrades performance for critical lunar and planetary observations. Even so, when well made a Schmidt or Maksutov will deliver very fine images of a wide variety of celestial objects.
In common with a refractor, the tubes of catadioptrics are sealed so that dirt and dust are largely excluded – a big plus for an instrument you’re going to take out into the country. But if you live in an area where dew is prevalent, some sort of collar or extension to prevent misting of the exposed corrector plate at the front of the tube is a good idea. In practice, many people seeking a highly versatile, very portable (for the aperture) scope that can be used for all sky subjects and astrophotography will tend to opt for some form of compound instrument. In short, they’re excellent general-purpose scopes.

Telescope Mounts
The best telescope in the world – be it refractor, reflector, or catadioptric – will be rendered useless unless it’s attached to some form of stable mount that permits it to be directed to any desired part of the sky and to follow a celestial object smoothly and precisely. A stable mount is one that, while you’re using a moderate to high power, will accept a small rap to the tube and the vibrations will die away in about a second. While there are variants on a theme, you’ll encounter two types of mount: altitude-azimuth (or ‘alt-az’) and equatorial. Alt-az mounts operate like camera-style pan and tilt heads, moving the scope up and down (in altitude) and left to right (in azimuth). Equatorial mounts also possess two axes, but one is aligned with the rotational axis of the Earth.

Those intending to use small telescopes for casual viewing or daytime use (say, bird watching) will find the alt-az mount preferable. Better engineered mounts of this type will be equipped with finely threaded slow-motion controls that enable the scope to be moved smoothly by tiny amounts, especially when using high power. The value of such refinements will be all too apparent when tracking a star or planet at higher magnifications. The aforementioned Dobsonian is a form of alt-az mount that’s quite popular. Unlikely telescope making materials such as MDF and Teflon feature in its construction, which results in a lightweight, low-centre-of-gravity mount that glides smoothly about both axes with fingertip control. A Newtonian reflector mounted in this fashion is not only extremely easy to set up and intuitive to use for the novice, but very good value, too.

For a telescope dedicated to astronomical use and where astrophotography is a future prospect, prime consideration must be given to some form of equatorial mount in order to automatically counteract the effects of the Earth’s rotation. It’s far easier to track a celestial object with a scope mounted in this fashion since the observer need only concern themselves with turning the scope about one axis, not two simultaneously as in the alt-az. When properly set up, turning the slow-motion control of the equatorial’s polar axis is all that’s required to keep an object in view. More sophisticated mounts will have built-in electric motor drives to do this, freeing you to concentrate on observing. Another big plus of the equatorial is that it’s frequently equipped with graduated circles about both axes that permit you to “dial-in” the co-ordinates of celestial objects found in books and magazines.

So is the equatorial mount best? For the casual observer who desires a highly portable scope mount that permits them to set up in a variety of locations and be observing in minutes, then an alt-az is actually preferable – especially a Dobsonian. Equatorials, while virtually mandatory for most forms of astrophotography and critical observations of the Moon and planets at high power, do need to have their polar axes precisely aligned with the rotational axis of the Earth in order to be used for casual astronomy. While polar alignment is not a particularly difficult procedure that becomes easier with practice, it can take a little time at the start of your observing session. This is why large instruments on equatorial mounts are usually permanently housed in observatories.

‘Go To’
Currently very much in vogue are the computer-controlled ‘Robo-Scopes’ appearing on the market in various guises from several manufacturers. These are refractors, reflectors, and catadioptric instruments on mounts that are controlled either by a plug-in hand-held computer or remotely by an external PC. This allows you to direct the scope to any object in the computer’s database. At first glance these Go To units would appear to be the answer to a novice’s dream, since they ostensibly take all the hard work out of finding elusive objects like faint galaxies, star clusters and minor planets.

There’s no denying that when well-engineered, these robotic scopes are great fun to use, as they almost magically slew across the sky in search of whatever you’ve keyed in, zeroing in on the target to be presented in the eyepiece. The downside (yes, there always is one!) is that the technology is not quite at the stage where these scopes will automatically orient themselves when you take them outside and switch them on. Almost all Go To systems will ask you to enter the precise geographical location of your viewing site (or the nearest city) and the date and time at the beginning of each observing session. This enables the instrument’s onboard computer to accurately calculate the positions of any celestial objects you may care to look at.

Usually you’ll also have to level the telescope’s tube and point it north (or south, in the Southern Hemisphere), then launch into an alignment procedure that uses two bright stars (which you must know by name!) to synchronise the telescope’s co-ordinate system with that of the sky. These setup routines are easily mastered with practice, but it has to be said that if programming a video recorder gives you problems, then the vast majority of the current batch of robotic scopes will frustrate you. Not only that, the money spent on a Go To’s electronic mount could be invested in a traditionally mounted scope of larger aperture.

Finders
When used at medium to high powers any telescope will show you a small window on the sky. This can make finding faint objects a frustrating process without the aid of a finder. As the name suggests, this is an observing aid that assists you in locating celestial objects and all scopes – irrespective of type or design – should be equipped with one. In their commonly encountered form they resemble a miniature scope attached by a bracket close to the eyepiece of the main instrument. They have low magnifications with wide fields of view and are equipped with a cross hair or reticule at the focus similar to a gun sight. If it’s correctly aligned with the main scope, centring the object in the finder gets it in the big instrument. Look for finders with apertures (front lenses) larger than an inch (25-mm) wherever possible – larger finders give brighter images, making the location of fainter objects that much easier. A variant that’s very popular is the ‘laser’ or ‘reflex’ sight that projects a point of light against the background sky. Many people prefer this intuitively simple-to-use option, but you’re limited to naked eye objects with no magnification.

Everything Has Its Price
While it may be tempting financially; resist the urge to buy a cheap department store scope. Many of these instruments are of poor quality either optically or mechanically (frequently both) and will inevitably lead to disappointment. If you’ve a budget of less than £200, consider a good pair of binoculars instead. That said; there are a great number of quality instruments that can be obtained second-hand that an experienced member of your local astronomical club may be prepared to check out on your behalf. Or have you considered building one yourself? If you’re gifted with your hands and enjoy working in wood, then it’s quite easy to buy the optics and make a quality Dobsonian reflector yourself. Again, members of your local club will be able to help you with the details of construction.

Even if you’re a beginner fortunate enough to have a sizeable disposable income, don’t buy the largest, most expensive instrument you can find. If you’re just learning to identify your constellations, then many of the advanced features that these observatory-class instruments possess are not likely to be any use to you. It’s also well worth remembering that there’s more to a telescope than the tube assembly, mount, and tripod. Be sure to save some of your budget for additional eyepieces to expand the scope’s magnification range; filters for combating light pollution/enhancing nebulae and planetary images; and any number of other available accessories – particularly for astrophotography.

Concluding Thoughts
So, what is the best telescope for you? Quite simply, it is the one you’ll use most often. An optically-perfect but massive refractor will be effectively useless if you can’t lift it outside, and the largest Dobsonian will not show you the faintest galaxies if the only place you can use it is a light-polluted park in the centre of a city. Consider carefully what you feel to be your primary observing interest, where you’re likely to be able to observe and what constitutes ‘portable’ to you. Visit your local astronomy club – they’ll often have observing nights where you can use scopes and chat to their owners. A telescope is a big investment to most people and the universe is not going away, so take your time over the purchase. When you buy the instrument that’s right for you, you’ll possess the key to unlocking a universe of wonders.

Lenses
Most telescopes designed for beginners and intermediate users have the standard 1.25-inch focuser while the advanced and top-of-the-range instruments tend to have the 2-inch focuser.

For many years eyepieces were considered the poor relation of the telescope’s primary optics, but during the 1980s there was a revolution in eyepiece design that brought their resolution and image quality up to those of the telescope itself.  The basic eyepiece design was developed into many variations, but at I have found that three basic types of eyepieces serve all your needs.  They are Plössl, Super-Wide and Ultra-Wide.

And, if you own just a few eyepieces, the acquisition of a single Barlow lens will effectively double the number of powers of magnification you have available in your eyepiece collection.

Plössl eyepiece
The Plössl has four elements of two, almost identical, pairs of lenses. It has a wide field of view at about 50 degrees. It offers good contrast and colour correction and produces superb results in focal lengths between 30mm and 15mm with good eye relief and minimal aberration.

Super Wide Angle eyepiece
These 6-element eyepieces, with their 67° fields, offer exquisite wide-angle images at low-to-medium powers, ideal for star fields, galaxies, and other deep space observing. 60 – ­ 70 degree apparent field.

UltraWide Angle eyepiece
Ultra Wide Angle eyepieces permit extreme image fidelity and sharpness over an 84°apparent field of view.  For moderate-to-high-power applications Ultra Wide Angle eyepieces provide an uncommon observing experience. These are a good optical match with Maksutovs, Schmidt-Cassegrains, apochromatic refractors, and fast Newtonians.

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